Direct synthesis of oxalic acid via oxidative CO coupling mediated by a dinuclear hydroxycarbonylcobalt(III) complex

Oxidative coupling of CO is a straightforward and economic benign synthetic route for value-added α-diketone moiety containing C2 or higher carbon compounds in both laboratory and industry, but is still undeveloped to date. In this work, a rare coplanar dinuclear hydroxycarbonylcobalt(III) complex, bearing a Schiff-base macrocyclic equatorial ligand and a μ-κ1(O):κ1(O’)-acetate bridging axial ligand, is synthesized and characterized. The Co(III)-COOH bonds in this complex can be feasibly photocleaved, leading to the formation of oxalic acid. Moreover, the light-promoted catalytic direct production of oxalic acid from CO and H2O using O2 as the oxidant with good selectivity (> 95%) and atom economy at ambient temperature and gas pressure based on this dicobalt(III) complex have been achieved, with a turnover number of 38.5. The 13C-labelling and 18O-labelling experiments confirm that CO and H2O act as the sources of the -COOH groups in the dinuclear hydroxycarbonylcobalt(III) complex and the oxalic acid product.


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. Detection of H2O2 generated in the production of oxalic acid catalyzed by 3 (red), 4 (blue) and 5 (magenta) using iodometry method. S33 Supplementary Fig. 28. H2O2 detection based on neocuproine/CuSO4 titration: (a) UV-Vis spectra recorded for the titration of H2O2 standard samples; (b) UV-Vis spectra recorded for the titrations of H2O2 formed in the production of oxalic acid catalyzed by 3 (red), 4 (blue) and 5 (olive). S34 Supplementary Fig. 29. H2O2 detection based on Cerium sulfate titration: (a) UV-Vis spectra recorded for the titration of H2O2 standard samples; (b) UV-Vis spectra recorded for the titrations of H2O2 formed in the production of oxalic acid catalyzed by 3 (red), 4 (blue) and 5 (olive).

General information
Materials. All manipulations involving air-sensitive materials were performed under N2 atmosphere using standard Schlenk techniques or in gloveboxes. Chemicals were purchased from Sigma-Aldrich, Alfa Aesar, or J&K Scientific Ltd. and used without further treatment unless otherwise noted. Deuterated solvents (CD3OD, DMSO and CDCl3) were purchased from Cambridge Isotope Laboratory Inc, and used without further treatment. The macrocyclic ligand (H2L) was synthesized by literature procedures 1 .
Physical Measurements. 1 H NMR and 13 C NMR spectra were recorded on a Bruker Ascend TM 400 spectrometer at 298 K, and the chemical shifts were referenced to solvent residual signals. The common glass NMR tubes with an outside diameter of 5 mm (Wilmad WG-1000) were used for diamagnetic compounds, and J Young tubes (Wilmad GVA-5, with an outside diameter of 5 mm) were used for samples for which specific atmosphere were required. For 1 H NMR measurements, 0.0020 ~ 0.0030 mg of the sample was dissolved in 0.4 mL of deuterated reagents in the NMR tube and then the measurement was conducted; For 13 C NMR measurements, 0.0050 ~ 0.0100 mg of the sample was dissolved in 0.4 mL of deuterated reagents in the NMR tube and then the measurement was conducted.
The UV-vis measurements were conducted on a Shimadzu 2600 spectrometer. The infrared spectra were recorded using a Bruker Tensor 37 spectrometer. The XPS spectra were recorded on an ESCALAB 250 instrument. The X-band CW-EPR measurements were recorded on a Bruker EMX plus spectrometer at 97 K. Thermal analysis was carried out on Netzsch TG209 thermobalance under a N2 atmosphere. Magnetization measurements were carried out using a Quantum Design MPMS 3 SQUID magnetometer equipped with a 5T magnet. The elemental analysis results were obtained by a Vario EL cube analyzer. The GC measurements of liquid samples were performed on Shimadzu GC-2010 Pro instrument equipped with a flame ionization detector and a WondaCap Wax capillary column (30 m × 0.25 mm). Gas samples for GC measurements were analyzed using a Shimadzu 2014C equipped with a thermal conductivity detector and a custom column (Porapak-N 3.0 m×3.2 mm×2.1 mm). The formic acid product was determined and quantified by using a 400 MHz liquid NMR spectrometer. The amount of oxalic acid product was detected by LCMS, performed on Shimadzu LCMS-2020 instrument equipped with a UV detector using a Shim-Pack Scepter C18-120 reverse column (3 μm, 3×33 mm). The mobile phases were acetonitrile and water/5 mM NH4HCO3 solution, and the flow ratio was 5:95 at a total flow rate of 1.5 mL -1 min. In addition, oxalic acid was quantified using the timehonored method of precipitation by CaCl2. Light in the irradiation experiments were provided by a 500 W high voltage xenon lamp (CEL-HXF300-T3, Beijing China Education Au-light Co.,Ltd.,).
General procedure for the catalytic production of oxalic acid from CO 4.0 mL of methanol solution containing 0.0170 mmol of catalysts (complexes 1-5) was transferred into a 25.0 mL Schlenk flask. After three freeze-pump-thaw cycles, 1 atm of O2 was inflated into the Schlenk flask, followed by the addition of 1 atm of CO. The Schlenk flask was set 20.0 cm aside a 500 W xenon lamp at 30 ℃ for 28 h. 13 C-labelling experiment for the catalytic production of oxalic acid. 0.0170 mmol of complex 4-13 C was added into a 25.0 mL Schlenk flask with 4.0 mL methanol. After three freeze-pump-thaw cycles, 1 atm of O2 was inflated into the Schlenk flask, followed by the addition of 1 atm of 13 CO. The Schlenk flask was set 20.0 cm aside a 500 W xenon lamp at 30 ℃. After stirring the reaction mixture for 28 h, 0.680 mmol of CaCl2 was added to the reaction solution, and the resulted white precipitate was collected and used for IR analysis.

General procedure for product detection
Formic acid detection: 4.0 mL of methanol solution containing 0.0170 mmol of complex 4 was transferred into a 25.0 mL Schlenk flask. After three freeze-pump-thaw cycles, 1 atm of O2 was inflated into the Schlenk flask, followed by the addition of 1 atm of CO. The Schlenk flask was set 20.0 cm aside a 500 W xenon lamp at 30 °C for 28 h. Then, excess amount of NaOH was added to the reaction solution at room temperature in an air atmosphere, and then the reaction solution was dried and used for NMR analysis.
Oxalic acid detection: 4.0 mL of methanol solution containing 0.0170 mmol of complex 4 was transferred into a 25.0 mL Schlenk flask. After three freeze-pump-thaw cycles, 1 atm of O2 was inflated into the Schlenk flask, followed by the addition of 1 atm of CO. The Schlenk flask was set 20.0 cm aside a 500 W xenon lamp at 30 °C for 28 h. The precipitate was filtered off and the filtrate was directly analyzed by LCMS at room temperature in an air atmosphere. (The amount of oxalic acid product was detected by LCMS, performed on Shimadzu LCMS-2020 instrument equipped with a UV detector using a Shim-Pack Scepter C18-120 reverse column (3 μm, 3×33 mm). The mobile phases were acetonitrile and water/5 mM NH4HCO3 solution, and the flow ratio was 5:95 at a total flow rate of 1.5 mL/min.) CO2 detection: 4.0 mL of methanol solution containing 0.0170 mmol of complex 4 was transferred into a 25.0 mL Schlenk flask. After three freeze-pump-thaw cycles, 1 atm of O2 was inflated into the Schlenk flask, followed by the addition of 1 atm of CO. The Schlenk flask was set 20.0 cm aside a 500 W xenon lamp at 30 °C for 28 h. After the reaction, the gas is collected by balloon and then connected to GC injector for gas analysis.
Dimethyl oxalate, dimethyl carbonate and methyl formate detection: 4.0 mL of methanol solution containing 0.0170 mmol of complex 4 was transferred into a 25.0 mL Schlenk flask. After three freeze-pump-thaw cycles, 1 atm of O2 was inflated into the Schlenk flask, followed by the addition of 1 atm of CO. The Schlenk flask was set 20.0 cm aside a 500 W xenon lamp at 30 °C for 28 h. The precipitate was filtered off and the filtrate was then analyzed by GC.

Assignment of the spin state of Co(II)/Co(III) centers in complexes 1-2
The calculated spin density for 1 showed that 2.58 unpaired electrons were found S6 on each of the cobalt centers, consistent with the high spin (S = 3/2) state for both of the Co(II) centers. Unrestricted corresponding orbital analysis of 1 is also in line with the assignment of two high spin Co(II) centers. The spin density analysis of 2, as shown in Supplementary Fig. 4, showed that 2.61 unpaired electrons were located on the Co(II) center. Meanwhile, unrestricted corresponding orbital analysis of 2 confirms the presence of a total of three unpaired electrons on the Co(II) center. Given all the aforementioned results, the assignment of a high spin (S = 3/2) Co(II) center and a low spin (S = 0) Co(III) center was made.

Computational details
All calculations were performed on the ORCA quantum chemistry program package (version 5.0.3). The B97-3c calculation setup developed by Grimme and coworkers was applied 2 . This setup is based on the B97 GGA functional and includes D3 with three-body contribution and a short-range bond length correction. The modified, stripped-down triple-ζ basis, def2-mTZVP 3 is used in the setup. The solvation effect was considered using the conductor-like polarizable continuum model (C-PCM) 4 . Structure optimizations were performed without any geometrical constraint. For the relatively large complex 5 with a total of 174 atoms, which is nearly twice the size of other dicobalt complexes, the crystal structure with the optimized positions for hydrogen atoms was used for the unrestricted corresponding orbital and spin density analysis.

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The carbon-nitrogen bond lengths are used as the indexes to distinguish imine moietes (C=N bonds) from amine moieties (C-N bonds). Both of the imine and amine moieties are found in H2L ligand (CCDC 2235093), with bond lengths of 1.272-1.280 Å and 1.445-1.447 Å, respectively. For the dicobalt complexes 1-5, all the carbonnitrogen bond lengths are in the range of 1.280-1.293 Å, which are very comparable to the bond lengths of the imine groups in H2L ligand and typical values (1.280-1.284 Å) reported for other imine compounds 5 . In addition, the resonances of protons in both the amine (-CH 2 -NH-) and imine (-CH=N-) moieties were seen in the 1 H NMR spectrum of H2L (δ ~ 4.45 ppm for the 4 protons in amine moieties and δ ~ 8.63 ppm for the 2 protons in imine moieties, Supplementary Fig. 8). No -CH 2 -NH-signal was observed in 1 H NMR spectra of the obtained diamagnetic dicobalt complexes (Supplementary Fig. 10 and Supplementary Fig. 14). These results indicated that the amine moieties in the free base H2L ligand were converted to imine moieties during the metallation.
Without the addition of Co(OAc)2, no conversion or decomposition of the H2L ligand was observed under identical conditiaons to the synthesis of dicobalt(II) complex 1. To confirm the generation of H2 during the synthesis of complex 1, the reaction was performed in a sealed J. Young NMR tube in d6-DMSO at 80 °C. The recorded 1 H NMR spectrum of the reaction solution showed a singlet resonance at δ = 4.35 ppm that is characteristic for H2 ( Supplementary Fig. 1). Similiar cobalt mediated dehydrogenation of amines was also known 6 .
Procedure of the 13 C-labelling experiment: In a 25.0 mL Schlenk flask, a solution of complex 3 (0.0350 g, 0.0170 mmol) in 4.0 mL methanol was degassed by three freeze-pump-thaw cycles. 1 atm of O2 was inflated into the Schlenk flask, followed by the addition of 1 atm of 13 CO. The solution was stirred for 24 h at 50 ℃. The red precipitate was collected by filtration and sent for the IR analysis. Fig. 13. IR spectra of 4 prepared using 12 CO (red) and 13 CO (blue).
4.0 mL of methanol solution containing 0.0170 mmol of complex 4 was transferred into a 25.0 mL Schlenk flask. After three freeze-pump-thaw cycles, 1 atm of O2 was inflated into the Schlenk flask, followed by the addition of 1 atm of 13 CO. The Schlenk flask was set 20.0 cm aside a 500 W xenon lamp at 30 ℃ for 28 h. Then, excess amount of CaCl2 was added to the reaction solution, and the resulting white precipitate was collected and used for IR analysis. Similar shifting trend has also been reported for unlabeled and 13 C-labeled oxalates. Supplementary Fig. 24. IR spectra of Ca 13 C2O4 (a, obtained by adding CaCl2 to the reaction solution of the catalytic production of oxalic acid from 13 CO) and Ca 12 C2O4 (b). Supplementary Fig. 25. The MS measurements (negative mode) of H2C2O4 standard reagent (a), and H2C2O4 produced using H2 18 O/ 16 O2 (b) and H2 16 O/ 18 O2 (c).

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The formation of H2O2 in catalytic reaction was detected using the iodometry method 7 . The reaction solution was extracted with diethyl ether. The precipitate was filtered off and the filtrate was concentrated under reduced pressure. 3.0 mL of KI aqueous solution (0.17 M) was added to the concentrated solution and then filtered by Nylon membrane. The filtrate was used for the UV-Vis measurement. Supplementary Fig. 27. Detection of H2O2 generated in the production of oxalic acid catalyzed by 3 (red), 4 (blue) and 5 (magenta) using iodometry method.
(i) Neocuproine/CuSO4 titration based colorimetric method 8 . The reaction solution was extracted with diethyl ether. The precipitate was filtered off and the filtrate was concentrated under reduced pressure. 2.0 mL of the colorimetric titrant (6 mM neocuproine, 4.2 mM CuSO4, 25/75 (v/v) ethanol/DI water mixture) was added to the concentrated solution and then filtered by Nylon membrane. The UV-Vis spectrum of filtrate was then recorded. The absorbance at 454 nm was used to determine the H2O2 concentration based on the calibration curve. Supplementary Fig. 28. H2O2 detection based on neocuproine/CuSO4 titration: (a) UV-Vis spectra recorded for the titration of H2O2 standard samples. Arrow indicates the change of the absorption intensity with different [H2O2] (0, 0.004, 0.008, 0.012, 0.016, 0.020, 0.024, 0.028, 0.032, and 0.036 mM); (b) UV-Vis spectra recorded for the titrations of H2O2 formed in the production of oxalic acid catalyzed by 3 (red), 4 (blue) and 5 (olive).

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(ii) Cerium sulfate titration based colorimetric method 9 (2 Ce 4+ + H2O2 → 2 Ce 3+ + 2 H + + O2). The reaction solution was extracted with diethyl ether. The precipitate was filtered off and the filtrate was concentrated under reduced pressure. 2.0 mL of the colorimetric titrant (Methanol solution of cerium sulfate, 6 mM) was added to the concentrated solution and then filtered by Nylon membrane. The filtrate was then analyzed by UV-Vis spectrometer. The amount of H2O2 can be calculated as half of the consumed Ce 4+ (1 Ce 4+ ≈ 1/2 H2O2). The concentrations of Ce 4+ before and after the reaction were determined by UV-Vis spectrometer at a wavelength of 316 nm.

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Crystallographic data was collected on Rigaku XtalAB Pro MM007 DW diffractometer with graphite monochromated Cu Kα radiation ( = 1.54178 A). The diffractive measurement device type is 'XtaLAB AFC12 (RINC): Kappa single'. Structures were solved using direct method and then refined using SHELXL-2014 and Olex2 [10][11][12] to convergence, in which all the non-hydrogen atoms were refined anisotropically during the final cycles. All hydrogen atoms of the organic molecule were placed by geometrical considerations and were added to the structure factor calculation. For 1, 2, 3 and 5, we used the PLATON SQUEEZE procedure to remove the uncoordinated solvent molecules which could not be modeled properly 13 . Additionally, for 3, we refined the structure by using some necessary restrains of anisotropy, such as RIGU and SADI for the counter cation fragments. For 4, we refined the structure with the using of DELU command for the -COOH fragments. As a consequence of the packing forcing, the O7-H7 bond oriented to a direction that hampered the formation of a normal hydrogen bonding in 4. Additionally, we refined the structure by using SIMU command for 5 and omitting the some of the most disagreeable points. As a consequence of the packing forcing, the O15-H15 bond oriented to a direction that hampered the formation of a normal hydrogen bonding in 5.  (5) 16.0183 (7) 13.5945 (2) 29.4492 (2) 17.4462 (5) c/Å 16.2138(6) 18.0697 (7) 14.5111 (2)